FIG. 12A shows a schematic example of an antenna (refer to a patent literature 1). An antenna 100 is mounted on a folding type (clamshell type) cellular phone. The antenna 100 includes an upper side printed wiring board 101, an impedance element 102, a hinge 103, a lower side printed wiring board 104 and an impedance element 105.
The hinge 103 includes a mechanism to connect an upper side case 107 and a lower side case 108 of the cellular phone each other. Furthermore, the hinge 103 includes a metallic component 103a which is made of metal such as stainless steel.
An electric circuit (not shown in the figure) and a ground pattern (not shown in the figure), which functions as the ground of the electric circuit, are formed on the upper side printed wiring board 101. The upper side printed wiring board 101 is arranged within the upper side case 107.
The impedance element 102 is a circuit element which is arranged on the upper side printed wiring board 101. The impedance element 102 includes a resonant circuit 111 shown in FIG. 12B. One of both end parts 111a and 111b in the resonant circuit 111 connects with the ground pattern of the upper side printed wiring board 101. The other of both end parts 111a and 111b connects electrically with the metallic component 103a of the hinge 103 through a connection member 113 shown in FIG. 12A.
An electric circuit (not shown in the figure) and a ground pattern (not shown in the figure), which functions as the ground of the electric circuit, are formed on the lower side printed wiring board 104 similarly to the upper side printed wiring board 101. The lower side printed wiring board 104 is arranged within the lower side case 108. The impedance element 105 is a circuit element which is arranged on the lower side printed wiring board 104. The impedance element 105 includes the same resonant circuit 111 as the impedance element 102 includes. One of both end parts 111a and 111b in the resonant circuit 111 connects with the ground pattern of the lower side printed wiring board 104, and the other connects electrically with the metallic component 103a of the hinge 103 through a connection member 114.
According to the antenna 100, as mentioned above, the ground patterns of the printed wiring boards 101 and 104, the resonant circuits 111, which are included in the impedance elements 102 and 105, the metallic component 103a of the hinge 103 and the connection members 113 and 114 connect electrically each other. These connected components function as a dipole antenna 106.
By the way, the cellular phone is equipped with an antenna 110 in addition to the dipole antenna 106. The dipole antenna 106 functions, for example, as an antenna which receives a radio wave of the ground digital television broadcasting. The antenna 110 has a function to send and receive a radio wave of a frequency band (wireless communication frequency band) which is arranged in order to communicate with a base station by wireless.
According to the patent literature 1 (Japanese Patent Application Laid-Open No. 2009-147513), the impedance elements 102 and 105 have a function to prevent the dipole antenna 106 from causing a harmful influence on the antenna 110. That is, in the case that the dipole antenna 106 (each ground pattern of the printed wiring boards 101 and 104) resonates in the wireless communication frequency band, this resonance causes the harmful influence on the wireless communication between the base station and the antenna 110. According to the patent literature 1, to cope with the above-mentioned situation, the impedance elements 102 and 105 have a function to restrain that a electric current, whose frequency spectrum is in the wireless communication frequency band, flows (is induced) on the ground patterns of the printed wiring boards 101 and 104. Here, the electric current, whose frequency spectrum is in the wireless communication frequency band, may be denoted hereinafter as a base station communication current Ibe.
That is, the resonant circuit 111, which is included in the impedance elements 102 and 105, has a circuit 115 which becomes an open circuit for the base station communication current Ibe. For this reason, the impedance elements 102 and 105 attenuate the base station communication current Ibe. As a result, the impedance elements 102 and 105 prevent the ground patterns of the printed wiring boards 101 and 104 from resonating in the frequency band, which communicates with the base station by wireless, due to the base station communication current Ibe flowing. The above-mentioned function of the impedance elements 102 and 105 restrains the harmful influence, which is caused by the dipole antenna 106, on the antenna 110.
Here, the resonant circuit 111 furthermore includes a circuit 116 in addition to the circuit 115. The circuit 116 is designed so that the resonant circuit 111 may become a short circuit for an electric current of a frequency band to be assigned to the ground digital television broadcasting. As a result, the electric current, whose frequency spectrum is in the radio frequency band of the ground digital television broadcasting, is not attenuated by the resonant circuit 111, and then flows into the dipole antenna 106 (the ground patterns of the printed wiring boards 101 and 104, the impedance elements 102 and 105, the metallic component 103a of the hinge 103, and the connection members 113 and 114). For this reason, the dipole antenna 106 can function as the antenna which receives the radio wave of the ground digital television broadcasting.
For example, a radio frequency band of the ground digital television broadcasting is 470 MHz to 770 MHz. A radio frequency band of the wireless communication with the base station is 1920 MHz to 2170 MHz.
FIG. 13A shows a schematic example of another antenna (refer to a patent literature 2 (Japanese Patent Application Laid-Open No. 1996(H08)-186420)). An antenna 120 includes a ground substrate 121, a dielectric substrate 122 and a monopole element 123. The dielectric substrate 122, which is used only for the antenna, is arranged so as to be vertical to the ground substrate 121. The monopole element 123 is formed on the dielectric substrate 122. The monopole element 123 includes a first antenna element 124, a second antenna element 125 and a resonant circuit 126. The first antenna element 124 and the second antenna element 125 are printed on the dielectric substrate 122. One end 124a of the first antenna element 124 connects electrically with a feeding source 128. The other end 124b of the first antenna element 124 connects electrically with the second antenna element 125 through the resonant circuit 126. Length L1 of the first antenna element 124 is designed so that the first antenna element 124 may resonate at a second frequency F2. Moreover, length L3 (L3=L1+L2), which is corresponding to total length of the first antenna element 124 and the second antenna element 125, is designed so that a whole of the monopole element 123 may resonate at a first frequency F1 which is lower than the second frequency F2.
The resonant circuit 126 is, for example, a parallel resonant circuit shown in FIG. 13B. The resonant circuit 126 is formed on the dielectric substrate 122. One end 126a of the resonant circuit 126 connects with the other end 124b of the first antenna element 124, and the other end 126b of the resonant circuit 126 connects with the second antenna element 125. The resonant circuit 126 is configured so as to become an open circuit for a electric current of the second frequency F2. For this reason, the resonant circuit 126 attenuates the electric current of the second frequency F2.
According to the antenna 120, the electric current having the second frequency F2, which is fed by the feeding source 128, is attenuated by the resonant circuit 126. For this reason, it is restrained that the electric current having the second frequency F2 flows from the first antenna element 124 to the second antenna element 125. Consequently, the electric current having the second frequency F2 flows only into the first antenna element 124 out of the first antenna element 124 and the second antenna element 125. As a result, the monopole element 123 (in this case, the first antenna element 124) resonates at the second frequency F2, and a signal having the second frequency F2 is sent and received through the air.
In contrast, a electric current having the first frequency F1, which is fed by the feeding source 128, flows from the first antenna element 124 to the second antenna element 125 through the resonant circuit 126. As a result, the monopole element 123 (in this case, the first antenna element 124 and the second antenna element 125) resonates at the first frequency F1, a signal of the first frequency F1 is sent and received through the air.
That is, the antenna 120 enables the wireless communication in the first frequency band which includes the first frequency F1, and in the second frequency band which includes the second frequency F2.